I set up my power source to 4.2 V @ 0.1 A. If I connect a Li-ion battery I get a current of about 0.1 A. If I connect a resistor of about 40 Ω I get a current of about 0.1 A. But if I put it all together (connect battery through the resistor) I get a current of about 10 mA.
In the first place I just wanted to calculate what resistor should I choose for a charging current of 0.1 A. But now I'm confused. Can anybody explain it to me?
The charging current will be dependent on the state of charge of the battery (the voltage difference between the charger and the battery).
Use Ohm's law: I = (Vcharge-Vbatt) / R.
If the battery is nearly empty (around 3V), the current will be around 25mA and when it's nearly full it will be almost zero.
Using just a resistor is not a good way to charge Lithium Ion batteries. There is no protection against overcharging or overdischarging.
If you care for your battery and/or do not want to set fire to your house, use a proper charging IC. There is a reason these exist.
